The present invention relates generally to transmission line impedance matching and, more particularly, to actual determination of the line impedance of a transmission line for signaling at nontrivial rates.
Transmission line impedance matching is an important part of virtually every high-frequency circuit operation, including both analog and digital signal transmissions. For example, with the transmission of such signals over a simple conductor (which can be virtually any circuit bearing a signal-carrying path), the source and load ends (or xe2x80x9cnodesxe2x80x9d) of the conductor should be properly terminated in order to prevent intolerable overshoot, undershoot, and reflections. In the presence of impedance mismatch, the severity of these undesirable effects increases with the length of the conductor, and the rate at which data can be transmitted over the conductor becomes more limited.
The impedance of both the source and load ends should be matched to the characteristic impedance of the conductor. The output impedance of the transmitting node and the input impedance of the receiving node typically differ from the characteristic impedance of the interconnecting transmission line, or the conductive path, carrying the signal. When the transmission line is used for bidirectional signal communication, it will be appreciated that each of the terminating nodes acts as a source when the node is sending a signal and each acts as a load when the node is receiving a signal, and a single impedance alteration does not typically provide the correct impedance matching for both transmission directions.
A multitude of techniques have been used to address impedance mismatch issues for a variety of applications including high-quality cable-matched interfaces, radio-frequency couplers, and large data networks using unshielded twisted pair cable. Many of these applications involve the transmission of signals at speeds that are sensitive, or xe2x80x9chighxe2x80x9d relative to the application""s tolerance for the above-mentioned undesirable effects. Reliable signaling for these applications requires that the line drivers, the line terminators or both, be matched to the transmission line impedance. Because of the difficulty of determining the actual line impedance, the prevailing practice has been to specify values for the line, the drivers and the terminators independently and to trade off the loss of signal integrity from mismatches against the cost of reducing them. For example, in attempting to effect an impedance match between an I/O pin of an IC to a printed-circuit board conductor, it is common to approximate the nominal impedance of the conductor based on specified values of operation for the conductor and the circuits connected to either end, and then to terminate one of these ends with a resistor based on the approximated nominal impedance. This practice is disadvantageous, however, due to the inaccuracies ensuing from both the approximation and the specified values of operation.
Accordingly, there is a need for an improved transmission line impedance-matching technique that lessens the inaccuracies resulting from this trade off.
According to various aspects of the present invention, embodiments thereof are exemplified in the form of methods and arrangements concerning approaches for line-impedance matching that involve a more direct manner of determining the line impedance for calibration purposes. It has been discovered that calibration of such a line can be achieved by appropriately adjusting the impedance at the signal terminating circuit in response to driving the line to a steady-state voltage using a first current, and then releasing this current and therein presenting a transient on the line. By properly evaluating the voltage level of the line in response to the transient, the proper adjustment can be made to the impedance at the signal terminating circuit.
Another specific implementation is directed to a process of calibrating impedance of a line connecting first and second nodes, comprising: driving the line to a steady-state voltage using a first current having a magnitude greater than zero; subsequently driving the current to a zero magnitude from the first node and therein presenting a transient voltage on the line; at the first node, indicating a voltage level of the line in response to the transient, comparing the line voltage level to a reference voltage, and then adjusting the impedance at the second node.
The above summary is not intended to provide an overview of all aspects of the present invention. Other aspects of the present invention are exemplified and described in connection with the detailed description.